1. Field of the Invention
The invention is directed to an optical waveguide fiber for which total dispersion and total dispersion slope are controlled and more particularly to an optical waveguide fiber including length portions for which total dispersion is opposite in sign in to that of total dispersion slope.
2. Technical Background
Compensation of total dispersion is a protocol that was adopted early in the design of single channel high performance systems. To augment the beneficial effects of dispersion compensation for multi-channel wavelength division multiplexed systems, effectively extending the dispersion compensation over a desired operating wavelength band, the concept of dispersion slope compensation was introduced. Implementation of this concept has increased fiber span length between electronic regeneration modules in high performance systems into the range of hundreds of kilometers. System construction and maintenance cost has been significantly reduced.
Furthermore, to improve cable manufacturing efficiency and to reduce the cost of repair cable inventory, optical waveguide fibers were designed that compensated total dispersion within the length of a given waveguide fiber. This advance eliminated the need to select individual waveguide fibers, having particular values of total dispersion magnitude and sign, to achieve total dispersion compensation for a system.
In high data rate, long distance systems, it is desirable to compensate total dispersion slope within the waveguide fibers to allow for total dispersion compensation over a band of wavelengths, thus providing compensation for all wavelengths in wavelength division multiplexed systems.
Improvement in the properties of optical waveguide fibers are still being sought to further increase system capacity and to continue to reduce system cost. For example, it has been found that fiber lengths that exhibit negative total dispersion and negative total dispersion slope require the refractive index profile to exhibit a waveguide dispersion, which is a part of the total dispersion, having a steep waveguide dispersion slope. Such profiles tend to couple power into cladding modes and are sensitive to bend and micro-bend, all factors that reduce transmitted signal power. The profiles also are quite sensitive to ordinary manufacture variations. These and other problems are addressed by the present invention.
The following definitions are in accord with common usage in the art.
The refractive index profile is the relationship between refractive index or relative refractive index and waveguide fiber radius.
A segmented core is one that is divided into at least a first and a second waveguide fiber core portion or segment. Each portion or segment is located along a particular radial length, is substantially symmetric about the waveguide fiber centerline, and has an associated refractive index profile.
The radii of the segments of the core are defined in terms of the respective refractive indexes at respective beginning and end points of the segments. The definitions of the radii used herein are set forth in the figures and the discussion thereof.
Total dispersion, sometimes called chromatic dispersion, of a waveguide fiber is the sum of the material dispersion, the waveguide dispersion, and the inter-modal dispersion. In the case of single mode waveguide fibers the inter-modal dispersion is zero.
The sign convention generally applied to the total dispersion is as follows. Total dispersion is said to be positive if shorter wavelength signals travel faster than longer wavelength signals in the waveguide. Conversely, in a negative total dispersion waveguide, signals of longer wavelength travel faster.
The effective area is
Aeff=2xcfx80 (∫E2 r dr)2/(∫E4 r dr), where the integration limits are 0 to ∞, and E is the electric field associated with light propagated in the waveguide.
The relative refractive index percent, xcex94%=100xc3x97(ni2xe2x88x92nc2)/2ni2, where n, is the maximum refractive index in region i, unless otherwise specified, and nc is the average refractive index of the cladding region. In those cases in which the refractive index of a segment is less than the average refractive index of the cladding region, the relative index percent is negative and is calculated at the point at which the relative index is most negative unless otherwise specified.
The term xcex1-profile refers to a refractive index profile, expressed in terms of xcex94(b) %, where b is radius, which follows the equation,
xcex94(b)%=xcex94(b0)(1xe2x88x92[¦bxe2x88x92b0¦/(b1xe2x88x92b0)]xcex1), where b0 is the point at which xcex94(b)% is maximum, b1 is the point at which xcex94(b)% is zero, and b is in the range bixe2x89xa6bxe2x89xa6bf, where delta is defined above, bi is the initial point of the xcex1-profile, bf is the final point of the xcex1-profile, and xcex1 is an exponent which is a real number.
A waveguide fiber telecommunications link, or simply a link, is made up of a transmitter of light signals, a receiver of light signals, and a length of waveguide fiber having respective ends optically coupled to the transmitter and receiver to propagate light signals therebetween. The length of waveguide fiber can be made up of a plurality of shorter lengths that are spliced or connected together in end to end series arrangement. A link can include additional optical components such as optical amplifiers, optical attenuators, optical switches, optical filters, or multiplexing or demultiplexing devices. One may denote a group of inter-connected links as a telecommunications system.
One aspect of the present invention is a controlled dispersion optical waveguide fiber including a core region and a clad layer. The optical waveguide fiber has a number n of length portions each of which has a characteristic refractive index profile. That is, the optical waveguide fiber in accord with this aspect of the invention has a refractive index profile that varies along the fiber length. The refractive index profiles of the respective length portions are chosen to exhibit a particular total dispersion, D1, and total dispersion slope, Si, where i is an integer from 1 to n, the integer 1 assigned to a first length portion, the integer 2 to the second length portion, and so on, with the integer n assigned to the last or nth length portion. The total dispersion and total dispersion slope over the entire length of the optical waveguide fiber are controlled to within desired upper and lower limits by appropriate selection of the respective signs of total dispersion and total dispersion slope of the length portions. This aspect of the invention includes length portions having refractive index profiles that exhibit a sign of total dispersion opposite the sign of the associated total dispersion slope.
In an embodiment of this aspect of the invention, every length portion has total dispersion opposite in sign to its total dispersion slope.
In another embodiment of this aspect of the invention the optical waveguide fiber includes length portions having total dispersion and total dispersion slope of the same sign.
A second aspect of the invention is a telecommunication system including an optical waveguide fiber made in accord with the first aspect of the invention or any of the embodiments thereof.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.